One type of thread milling cutter has a holder with a shank that is rotated by a milling machine, and that holds an insert with several projecting teeth for cutting threads in a workpiece. As the holder is rotated in a milling machine, it is pressed against a cylindrical region of a workpiece to be threaded, with the workpiece being slowly turned and shifted axially so after one turn of the workpiece a continuous thread with many turns has been cut therein. One type of thread milling insert (FIG. IA) is in the form of a largely rectangular flat plate with thread cutting teeth formed along one edge and with the opposite edge being straight. A flat surface of the insert is placed against a flat positioning surface on the holder and a two-directional clamping screw is installed to press the smooth edge of the insert against a secondary locating surface on the holder. The Snap-Tap brand thread milling insert is of this construction. The Vardex thread milling cutter sold by V.N.E. Corporation has smooth edges forming a V to accurately locate the insert in two directions.
The above-described prior art thread milling inserts have two cutting edges, one at the top edge of the cutting teeth and the other at the bottom edge of the cutting teeth. When one edge becomes worn after considerable use, the insert is removed, flipped over, and reinstalled to provide a second cutting edge. It might be thought that the locating edge could be formed with teeth so as to provide four cutting edges, but the repeated pounding to which the insert is subjected would result in damage to such teeth along the locating edge and/or to the positioning surface of the tool holder. Such damage must be avoided, to assure that each time an insert is installed its cutting edges lies within a small tolerance such as 0.0005 inch of the preceding one to minimize down time on CNC equipment. A threading tool with an insert having four separately useable cutting edges, which could be quickly relocated each time with its thread cutting edge located with high precision, would be of considerable value.